Method for determining resin viscosity with ultrasonic waves

ABSTRACT

A method for determining the dynamic viscosity of a specimen of a polymeric resin which is subjected to a time-varying temperature includes the steps of passing an ultrasonic sensing wave of known amplitude through the specimen, sensing the amplitude of the wave after it has travelled through the specimen, and from the degree of amplitude attenuation, obtaining a value which has a linear relationship to the logarithm of the instantaneous dynamic viscosity of the resin.

This application is a division of application Ser. No. 281,877, filedJuly 9, 1981, now U.S. Pat. No. 4,455,268.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of process controltechnology, and, more particularly, it relates to an automated systemfor controlling the cure cycle for structures or parts made of fiberreinforced composite material and the like, in which the physicalcharacteristics such as temperature and viscosity of the part itself,are used as active process control parameters.

2. Brief Description of the Prior Art

As their name implies, fiber-reinforced composite materials (also knownsimply as "composites") comprise a base or substrate material, such asan epoxy resin, which is impregnated, for structural strength, withfibers of such materials as carbon, graphite, glass, boron, and nylon.Composites typically exhibit extremely high strength-to-weight ratios,and, accordingly, their use is becoming increasingly popular in theaerospace industry.

One problem still associated with composites is the relatively high costof fabricating structural parts, such as, for example, aircraftfuselage, wing, and tail sections, from composite materials. Asignificant factor in the cost of fabricating such parts lies in thecare required to control the shape and thickness of the part throughoutthe fabrication process, while also achieving the necessary degree ofstructural integrity and strength of the part. Accordingly, new meansare constantly being sought for achieving these ends more efficientlyand at lower cost.

In fabricating a structure, such as an aircraft part, from compositematerials, usually the part is first shaped by laying a preselectednumber of layers of raw or partially cured composite material on a mold.When the desired shape is achieved, the composite is subjected to afinal curing process by placing the part in a pressurized oven known asan autoclave. In the autoclave, polymerization of the resin substrate iscompleted so that the molded shape is made permanent and the compositematerial is made hard and durable. Strictly speaking, the final curingprocess usually completes the final cross-linking of the prepolymerizedresin substrate.

Controlling the curing process presents some difficult problems. Forexample, the viscosity of the resin substrate changes during the curingprocess and, at times, the viscosity is low enough so that the resin isquite capable of flowing. Controlled flow of the resin is desired atcertain times during curing to achieve the required part shape thicknessand a structural strength. However, if the resin is allowed to flow inan uncontrolled manner, undesired micro-void formations and/orvariations in the thickness of the part can occur. These anomalies canbe minimized or controlled by appropriate applications of pressure andtemperature during the cure cycle.

For any given part geometry, the flow of the resin is determined by itsviscosity and ambient pressure. The viscosity of the resin, in turn, isa function of temperature and of the time the resin has already beensubjected to the final curing process. Thus, as temperature and pressureare varied during the curing cycle, the resin will undergo changes inviscosity. For each particular resin, and given temperature variationduring the curing cycle, it is possible to empirically determine a"viscosity behavior profile" or "VBP." Given the empirically-derived VBPfor a particular resin, it is possible to theoretically predict theviscosity of that resin at any time during the curing cycle. Inaccordance with the foregoing, present day curing process controltechniques monitor temperature and pressure during the cure cycle andmanually adjust these parameters in the expectation of achieving aviscosity at any point in time which closely matches the viscositypredicted on the basis of the VBP.

The above noted prior art method of controlling the curing process hasshortcomings, however. More particularly, it has been found that theactual viscosity of the resin during the curing cycle often does notparallel closely the expected viscosity behavior profile. Thisaberration is due to a variety of factors, the most notable ones beingvariations in the moisture content of the resin and in the productiontechniques used to make the pre-impregnated resin ("pre-preg").Furthermore, the final polymerization and cross-linking reactions of theresin which ideally occur only during the final curing step, also occur,albeit at a slow and varying rate, in the pre-impregnated resin.Therefore, even resins having the same original chemical composition mayhave different states of polymerization when the final curing process isinitiated, and therefore will display somewhat different viscositybehavior profiles under the same cure process conditions. Accordingly, aprediction of expected viscosity during a particular point in the cureprocess based solely upon measurement of temperature and pressure yieldsresults that are, at best, poor approximations of the actual viscosityvalues.

Part thickness and resin/fiber ratio are functions of the flowingability of the resin. Furthermore, the flowing ability of the resin isrelated to its viscosity. Therefore, it can be appreciated that animproved ability to monitor or predict the viscosity of the resin duringthe curing process is likely to lead to improved ability to finelycontrol part thickness and resin fiber ratios, and therefore thestructural strength of the composite parts.

Another factor having an effect on the structural characteristics of thefinished composite part is the porosity of the cured composite material.This porosity is a result of the presence of microscopic voids("microvoids") produced by the release of volatile materials (usuallywater vapor) during the cure process. It is usually desired to minimizethe porosity of the finished part, and this can be achieved throughjudicious application of pressure during the curing cycle. However, theeffectiveness of such application of pressure for the purpose ofminimizing the porosity of the composite material is dependent upon theactual viscosity behavior profile during the cure cycle. Moreover, sinceit is difficult to ascertain accurately the moisture content of thepre-impregnated resin prior to the initiation of the cure cycle, andsince the formation of microscopic voids in the materials during thecure cycle cannot be measured or monitored in any meaningful way, thepressure control of porosity during the curing cycle is likely to benon-optimal without means for substantially continuously monitoring theactual viscosity of the resin during the curing cycle.

In view of the foregoing, a two-fold need has been felt in the industryfor improved monitoring and control of the curing process. First, it hasbeen a desired goal in the industry to monitor actual temperature,pressure and most importantly, viscosity at various points on the partat various times during the curing cycle, and to control the appliedtemperature and pressure in accordance with a comparison between adesired temperature, pressure and viscosity profile and the actual, orsensed, profiles. Second, there has been a need for reliably minimizingthe development of microvoids in the part as the part is cured.

With regard to the viscosity monitoring function, one method that hasbeen explored in the prior art, measures the viscosity of the substratethrough changes in the substrate's dielectric properties. See, forexample, Mayberry, "Dielectric Cure Monitoring of Polyimides,1" publishdin In-Process Quality Control for Non-Metallic Materials by U.S. ArmyMaterials and Mechanics Research Center, Watertown, Mass. (1980). Whilethis method has shown some promise, it has yet to see significantcommercial application, due, in large part, to a lack of reliability,and unacceptably low signal-to-noise ratios.

With regard to the problem of minimizing the development of microvoids,little has been accomplished to date in the way of "in-process" methods.Rather, emphasis has been placed on non-destructive evaluation of thecured part to ascertain its porosity, and then adjusting the processingparameters based upon this "after-the-fact" evaluation. The mostpromising non-destructive evaluation technique involves the use ofeither acoustic emission analysis or acoustic stress wave factoranalysis. Both of these techniques make use of the fact that an analysisof the acoustic properties of the material can yield important datarelating to its structural characteristics. In the acoustic emissiontechnique, the sample is stressed or loaded in a predetermined way, andthe resultant low amplitude, ultrasonic noise is analyzed. See forexample Green and Landy, "Acoustic Emission NDE for Advanced CompositeStructures," Acoustic Emission Technology Corporation, 1979.

In the acoustic stress wave factor technique, an ultrasonic pulse istransmitted through the material for a given distance and then received.The received pulse is then analyzed, and the results of this analysisyield data related to the structural characteristics of the material.See, for example, Vary and Lark, "Co-relation of Fiber Composite TensileStrength with the Ultrasonic Stress Wave Factor," Journal of Testing andEvaluation, Volume 7, No. 4, (1979). While in-process use of theacoustic emission technique has been attempted, the need for stressingor loading the material while it is in the autoclave has made thepractical application of this method difficult.

Accordingly, the need has been felt in the industry for a method foraccurately and non-destructively monitoring the viscosity of thesubstrate during the curing cycle, and for using the results of theviscosity measurements for interactively controlling autoclavetemperature and pressure. A further need has been felt for a practical,in-process mechanism for controlling or minimizing the formation ofmicorvoids so as to produce composite parts of low porosity.

SUMMARY OF THE INVENTION

Broadly, the present invention comprises an automated system forcontrolling the curing process of composite structures through aninteractive, computerized feedback system. The system which interfacesdirectly with existing autoclave equipment, includes sensors foraccumulating real time information regarding ambient and part surfacetemperature, ambient pressure, and the visco-elastic state of thecomposite material during the curing process. The measured values ofthese parameters are compared to pre-determined optimal values, andcorrections are then made to the operational parameters to achieve anoptimal curing process and thereby desired specifications of curedcomposite structure.

Specifically, the control system of the present invention comprises atemperature-regulating mechanism and dual, interactive pressureregulating mechanisms. With regard to the temperature-regulatingmechanism, the temperature at various locations on the surface of thestructure being fabricated (i.e., the "part temperature") issubstantially continuously compared to a specified part temperature-timeprofile. Next, the ambient autoclave temperature is measured andcompared with a specified maximum allowable autoclave temperature forthat specified part temperature. Finally, the maximum temperaturegradient across the part is compared to a specified maximum temperaturegradient. If any of these comparisons indicates that excessive heat isbeing applied to the part, controls are actuated to lower the autoclavetemperature. On the other hand, if the results of these comparisonsindicate that insufficient heat is being supplied to the part, thencontrols are actuated to increase the autoclave temperature.

With respect to the first of the dual pressure-regulating mechanisms,the ambient pressure inside the autoclave is measured and compared to areference pressure which is specified by a predetermined optimalpressure-versus-time profile. If the ambient pressure is greater thanthe reference pressure, a vent valve control mechanism is actuated torelease pressurizing gas from inside the autoclave until the referencepressure is reached. Likewise, if the ambient pressure is below thereference pressure, a valving mechanism is actuated to allow pressurizedgas to enter the autoclave to raise the pressure to the desiredreference value.

The second pressure control mechanism operates by monitoring thevisco-elastic properties of the composite material as it is cured andthen comparing the measured values with a pre-selected viscositybehavior profile or to a preselected constant reference viscosity value.This is accomplished by introducing an ultrasonic wave into the partbeing cured and computing the resultant acoustic or ultrasonic wavefactor by measuring the amplitude attenuation or velocity of theultrasonic waves. From the measured wave factor at any point during thecure cycle, the viscosity of the material is derived and compared to areference value taken at that particular temporal point in apre-selected optimal viscosity behavior profile or the preselectedconstant reference viscosity value. If the measured viscosity is at theappropriate reference value and if the first pressure control mechanismso dictates, a valving mechanism can be actuated to increase the ambientpressure in the autoclave to achieve a pressurization which isconsistent with a desired resin flowability at the temperaturecorresponding to the reference viscosity value. Likewise, if themeasured viscosity is lower than the reference value, the pressurizationof the autoclave can be delayed to allow the viscosity to achieve thedesired value before pressurization begins.

The system utilizes temperature-sensitive means such as thermocouplesplaced at various locations on the surface of the part being cured andat various locations within the autoclave to measure part temperatureand autoclave ambient temperature, respectively. Pressure measurementsare taken by means of pressure transducers located inside the autoclave.The acoustic or ultrasonic wave factor is obtained, as previouslymentioned, by means of an ultrasonic wave generating apparatus utilizingtransmitting and receiving acoustic transducers located at variouslocations on the surface of the part, the received acoustic signal beingconverted by appropriate signal treatment means into an electricalsignal indicative of the wave factor.

The analog electrical signals indicative of part temperature, autoclavepressure, and wave factor are fed into a data acquisition and controlunit which converts the analog signals to digital signals for input intoa data processing unit. The data processing unit includes a memory whichhas stored within it the desired part and ambienttemperature-versus-time profiles, and the required pressure-versus-timeprofile, as well as the desired viscosity behavior profile (VBP) or thepreselected constant reference viscosity value. The VBP which is areference parameter of resin viscosity as a function of time andtemperature, or the constant reference viscosity value as applicable,are selected on the basis of the chemical composition and visco-elasticcharacteristics of the composite resin prior to the final curingprocess, and also on the basis of the desired physical characteristicsof the finished part. The data processing unit is also provided with aclock or timing apparatus so that the values of the operationalparameters can be measured in a real time mode. The memory also containsthe operational software with which the data processing unit isprogrammed. The stored software includes a process control algorithmwhich directs the system through the procedural sequence summarizedabove, making the necessary comparisons and computations as required ineach step of the sequence. Digital output signals are generated by thedata processing unit which are fed back into the data acquisition andcontrol unit, where they are converted to analog signals to which theautoclave temperature and pressure controlling mechanisms respond. Adata display unit is connected to the data processing unit and mayinclude a CRT terminal for real time readout of the operationalparameters as well as a printer and/or plotter for hard copy output.

As will be appreciated from the foregoing, and from the detaileddescription that follows, the present invention provides severalsignificant advantages over prior art process control systems. Forexample, the present invention allows control of the autoclave inputparameters (i.e., pressure and temperature) based upon a reliable andaccurate monitoring, rather than mere predictions, of the desired outputparameter (i.e., viscosity). Moreover, by use of real time monitoring ofthe visco-elastic properties of the composite material, and the realtime control of autoclave pressure based upon the results of suchmonitoring, optimal pressure conditions can be reliably obtained forminimizing the formation of microvoids during the curing process. Thedual pressure controlling mechanism of the invention allows the controlof pressure on the basis of both an empirically derived pressure profileand a specified viscosity behavior profile, thereby increasing theaccuracy and reliability of the pressure control mechanism. In addition,the ability to measure both part temperature and autoclave ambienttemperature provides added flexibility for interactive operator controlon the basis of maximum temperature overshoot, as well as part surfacetemperature differentials.

These and other advantages of the invention will be apparent from thedetailed description and appended drawing Figures which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a linear relationship between the attenuationof an ultrasonic wave in a composite structure and the logarithm of theviscosity of the composite;

FIG. 2 is a schematic view of an ultrasonic wave attenuation measuringdevice;

FIG. 3 is a graph showing an actual viscosity-versus-time andtemperature-versus-time profile of the final curing step of a compositestructure;

FIG. 4 is a schematic view of a preferred embodiment of the system ofthe present invention, and

FIG. 5 is a schematic representation of the process control logic of thepreferred embodiment of the system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following specification taken in conjunction with the drawings setsforth the preferred embodiment of the present invention in such a mannerthat any person skilled in the arts relating to the curing of compositematerials and in the electronic arts can use the invention. Theembodiment of the invention disclosed herein is the best modecontemplated by the inventors for carrying out their invention in acommercial environment, although it should be understood that variousmodifications can be accomplished within the parameters of the presentinvention.

Referring now principally to FIG. 1, it has been discovered inaccordance with the present invention that the ultrasonic waveattenuation (a.u.) of a composite structure which undergoes a finalcuring step, has a direct mathematical relation to the dynamic viscosity(η*) of the composite structure. As it was discussed above in theintroductory section of the present application for patent, the dynamicviscosity is a very important parameter of the curing process.Non-destructive and substantially continuous monitoring of the dynamicviscosity of the composite structure during the curing process has beena desired, but hitherto elusive goal of the composite manufacturingarts. For the sake of brevity, in the ensuing description, the termcomposite is used, unless otherwise indicated, to denote a compositestructure of a specific shape and composition which is undergoing afinal curing step in an autoclave or like device under application ofheat, and possibly pressure as well.

More particularly, it has been discovered in accordance with the presentinvention that the attenuation of an ultrasonic wave (a.u.) which isinduced in the composite by state of the art instruments while thecomposite is undergoing the final curing process, bears a linearrelationship to the natural logarithm (ln) of the dynamic viscosity (η*)of the composite. This linear relationship exists in a range ofviscosity and ultrasonic wave attenuation which is substantially in theentire working range of the curing process of the composite.

The linear relationship is amply shown on the graph of FIG. 1, whereinthe logarithm of the dynamic viscosity (η*) of the composite, expressedin poise units, is plotted against the amplitude of the ultrasonic wavewhich is measured by suitable receiver sensors while the incomingultrasonic wave amplitude is kept constant. Those skilled in the arts ofphysics, will readily appreciate that under the above-noted conditions,the measured root mean square amplitude values are linearly proportionalto the amplitude attenuation (a.u.).

The points indicated on the graph of FIG. 1 represent actualmeasurements of ultrasonic wave amplitude, and of viscosity which wasmeasured by standard rheological control methods. The straight line ofthe graph was drawn by application of the mathematical least squaremethod so as to best approximate the measured values. The linearrelationship shown on FIG. 1 is expressed by Equation 1

    Lnη*=M(a.u.)+C                                         [Equation I]

wherein η* and a.u. have the hitherto defined meanings, and M and C areconstants. More specifically, M is termed a gain coefficient which isconstant for a specific fiber and resin containing material, andgeometry or shape of composite structure. C is termed a geometric offsetfact or which is constant for a specific orientation of the sensorsutilized for the determination of the a.u. on a specific compositestructure.

It follows from the foregoing, that once the values of M and C areempirically determined for a specific composite having a specificorientation of associated ultrasonic sensors, thereafter the dynamicviscosity of the composite may be measured by simply measuring the a.u.of the particular composite-sensor combination. The above describedprinciple thus allows direct, non-destructive measuring or continuousmonitoring of the dynamic viscosity of a composite as the compositeundergoes a final curing process.

The enclosed table (Table I) shows the results of a computer performedmultiple regression analysis on the measured values of a.u. and therheologically measured values of viscosity. Table I shows that theabove-noted linear relationship between a.u. and ln η* is very good, andthat in the example specifically disclosed in Table I and shown in FIG.1, the constant M has a value of 0.99354 and C has a value of 3.7138.

                  TABLE I                                                         ______________________________________                                                               natural logarithm                                                             of dynamic viscosity                                   natural logarithm of   calculated by Equation                                 dynamic viscosity      I wherein M and                                        measured by standard   C have the below                                       rheological method     defined numerical                                      (ln η*)     a.u.   values                                                 ______________________________________                                        4.02535         0.35   4.0656                                                 4.12713         0.39   4.1058                                                 4.18965         0.40   4.11586                                                4.44265         0.62   4.33699                                                4.68213         0.99   4.70889                                                5.0814          1.57   5.29187                                                5.23644         1.62   5.34213                                                5.51343         1.77   5.4929                                                 5.65249         1.86   5.58336                                                6.65929         2.86   6.58851                                                7.00307         3.26   6.99056                                                7.0000          3.26   6.99056                                                ______________________________________                                         M = 0.99354                                                                   C = 3.7138                                                               

Referring now to the schematic view of FIG. 2, a state of the artultrasonic wave attenuation measuring device 10 is shown. The device 10is commercially available as model no. 206AU of the Acoustic EmissionTechnology Corporation of Sacramento, Calif. It should be noted for thesake of clear understanding of the present invention that while theultrasonic wave attenuation measuring device 10 is known in the art, andtherefore need not be described here in detail, the use of the measuredultrasonic wave attenuation for determining dynamic viscosity is new,and constitutes an important aspect of the present invention.

Thus, the ultrasonic wave attenuation measuring device 10 includes acontinuous wave oscillator 12 coupled to an ultrasonic pulser 14 whichis coupled to a sending transducer 16. The sending transducer 16 iscoupled to the surface 18 of the composite or specimen 20. A receivingsensor or transducer 22 is also coupled to the surface 18 of thespecimen 20. Signals emanating from the receiving transducer 22 areamplified in an amplifier 24, are counted in a counter totalizer 26, andmay be displayed in a suitable data display or fed into a computer. Theultrasonic wave attenuation measuring device 10 also includes athreshhold control 28 and a suitable reset clock 30. The device 10 iscapable of operating in the frequency range between approximately 30 KHzand 2.5 MHz and in the 20-190 decibel range. The above disclosed processfor measuring the viscosity by measuring ultrasonic wave attenuation inthe composite is preferably practiced, however, with ultrasonic waves inthe 100 KHz to 2.5 MHz range.

Referring now to the graph of FIG. 3, an actual viscosity-versus-time,and temperature versus time plot or profile of a final curing step of acomposite, is shown. The graph shows changes in the dynamic viscosity ofthe composite as a function of time while the composite is cured in anautoclave. The dynamic viscosity of the composite was measured inaccordance with the novel method of the present invention, i.e. bysubstantially continuously measuring the ultrasonic wave attenuation ofthe composite during the curing cycle. Because, the ambient temperature,as well as the pressure, is not constant in the autoclave during thecuring cycle, the ambient temperature in the autoclave was measured andrecorded simultaneously with the recording of the viscosity values.Furthermore, a point in the graph indicates the time when substantiallylarger than atmospheric pressure was applied.

The graph of FIG. 3, grossly shows that the viscosity of the composite,or more strictly speaking of the resin substrate of the composite,substantially continuously decreases during approximately the first halfof the curing cycle. This is in response to the substantiallycontinuously increasing temperature of the autoclave. In the approximatesecond half of the curing cycle, however, the viscosity increasessubstantially continuously even though the temperature is high, (atleast 240° F). This is because of the chemical cross-linking or finalpolymerization which occurs in the resin substrate of the composite andwhich drastically changes the visco-elastic properties of the composite.

As is known in the art, however, the above summarized, gross monitoringof the curing process is insufficient for attaining a fine control ofthe final properties of the cured composite. The graph of FIG. 3 alsoshows that relatively small changes in the autoclave temperature resultin significant changes in the viscosity of the composite. As it waspointed out above in the introductory section of the present applicationfor patent, finely controlling the relative in the parameters of thecuring cycle, and accurately determining the time when pressure shouldbe applied to the autoclave, has been an elusive goal of the prior art.In accordance with the present invention, this is, however, renderedpossible by the system 32 schematically shown on FIG. 4.

The system 32 includes an autoclave 34 of the type normally used in theart for the final curing of composites. A specimen 20 of the compositeis shown in the autoclave 34. The autoclave 34 includes a suitableheating element 36, cooling coils 38, and valves for introduction ofpressurized air, and pressurized nitrogen gas, into the autoclave 34.The valves for air and nitrogen respectively bear the reference numerals40 and 42, and a valve for venting pressurized gas out of the autoclave34 bears the reference numeral 44. A fourth valve 46 is provided forconnection to a vacuum line (not shown) whereby air or nitrogen gas canbe very rapidly removed from a sealing enclosure or "bag" (not shown)which typically encompasses the part being cured.

A plurality of heat sensors or transducers 48 are mounted into theautoclave 34 for sensing the ambient temperature of the autoclave 34.These are hereinafter referred to as ambient temperature sensors 48. Inaddition, a plurality of heat sensors or transducers 50 are also mountedin contact with the surface of the specimen 20 for sensing the actualtemperature of the specimen 20. The heat sensors or transducers 50 arehereinafter referred to as surface or part temperature sensors 50.

A plurality of ambient pressure sensors or transducers 52 are alsomounted in the autoclave 34. Finally, an ultrasonic wave generating andattenuation measuring device 10 is included in the system 32, and has anultrasonic wave sending transducer 16 and at least one receivingtransducer 22 mounted to the specimen 20 in the autoclave 34. In theherein described preferred embodiment of the system of the presentinvention, the ultrasonic shock wave attenuation measuring device 10 isthe above noted 206 AU model of the Acoustic Emission TechnologyCorporation of Sacramento, Calif.

The system 32 further includes a data acquisition and control module 54wherein analog signals incoming from the respective ambient and surfacetemperature sensors 48 and 50, pressure sensors 52, and ultrasonic waveattenuation sensors 22 are converted into digital signals. The digitalsignals of the data acquisition and control module 54 are then fed intoa central processing unit or computer 56, wherein the hereinafterdescribed calculations and comparisons are performed in accordance witha process control algorithm. The computer 56 includes a memory, intowhich the hereinafter specifically described reference data and profilesare fed.

Digital data corresponding to command controls emanating from thecomputer 56 are converted into analog control signals in the dataacquisition and control module 54. The analog control signals are usedto create the appropriate responses in the operation of the system 32,such as turning off or decreasing the output of the heating element 36,opening or closing the valves and the like. It should be noted that theindividual components of the system are of conventional construction,and, therefore, need not be described here in detail. In the hereindescribed preferred embodiment, the data acquisition and control module54 is a Hewlett Packard Model No. 3497A, the computer 56 may be anycommercially available computer compatible therewith. The system 32 alsoincludes a data display terminal 58 connected to the computer 56, whichmay include a CRT display (not shown) and a printer or plotter (notshown) for permanent recordation of the measured parameters of thecuring process.

Referring now to the schematic view of FIG. 5, the specific operationallogic of the preferred embodiment of the system is disclosed.

The system includes a temperature control logic which is governed by thedesired temperature profile of the curing process for the composite 20.This temperature profile is stored in the memory of the computer 56.More particularly, the entire curing process is divided into specificsteps which are sequentially numbered. For each step, the desired rateof heating expressed in units of °F./min., the desired constanttemperature of the composite (hold temperature) if any, and the desiredtime at the hold temperature (hold time) is programmed into the computer56. In addition, the maximum allowable heating rate, and a minimumheating rate, are also programmed into the computer. A specific exampleof the above data is shown in Table II, under the heading "Heat LogicParameters."

                  TABLE II                                                        ______________________________________                                        HEAT LOGIC PARAMETERS                                                                            Desired Part                                                                  Temp. at                                                          HEAT RATE   beginning of                                               STEP # °F./MIN                                                                            the step   HOLD TIME (MIN)                                 ______________________________________                                        1      8           200        0                                               2      3           250        0                                               3      0           250        60                                              4      3           350        0                                               5      0           350        120                                             6      -5           80        0                                               ______________________________________                                         MAXIMUM HEAT RATE °F./MIN: 10                                          MINIMUM HEAT RATE °F./MIN: 1                                           NUMBER OF HEAT LOGIC STEPS PROCESS STEPS: 6                              

The temperature control logic is also governed by data regarding themaximum allowable differential in temperature between any two pointsmeasured on the surface of the composite specimen 20. An example of theactual values of the aforesaid maximum allowable temperaturedifferential as a function of the actual average surface or parttemperature, is given in Table III under the heading "Temperature LagLogic Parameters."

The computer 56 is also programmed to observe a maximum tolerableovershoot temperature; i.e. a maximum permissible temperature differencebetween the required surface or part temperature and the average ambientautoclave temperature. In this regard it is noted, that the term averagetemperature is used to denote the average of the several simultaneousreadings of the several functioning temperature sensors 48 or 50, asapplicable, of the system 32.

                  TABLE III                                                       ______________________________________                                        TEMPERATURE LAG LOGIC PARAMETERS                                                     MAXIMUM PERMIS-                                                               SIBLE                                                                         PART TEM-                                                                     PERATURE                                                               STEP # DIFFERENTIAL    FROM TEMP   TO TEMP                                    ______________________________________                                        1      15               80         200                                        2      10              200         230                                        3      5               230         250                                        4      5               250         350                                        ______________________________________                                         MAXIMUM TEMP OVERSHOOT = 20                                              

Thus, in the example shown in Table III, the ideal parameters of thecuring process require, and the computer 56 is accordingly programmed sothat during the first step of the process, the maximum temperaturedifferential of the composite specimen 20 should not exceed 15° F., andin the second step the maximum temperature differential should notexceed 10° F. Furthermore, the ambient temperature of the autoclave 34must never exceed by more than 20° F. the computed required parttemperature for this time in the process cycle.

The heat sensors 48 and 50 of the system 32 continuously monitor theambient autoclave and the composite part temperature. According to thelogic programmed into the computer 56 in the form of the process controlalgorithm, the part temperature (T_(p)) is continuously compared by thelogic to the programmed temperature as shown in Table II. If the parttemperature is too high, the heating element 36 is shut off or decreasedin output. If the part temperature is low or at the programmedtemperature, the logic then compares the ambient temperature to themaximum allowable temperature corresponding to the then required parttemperature. In other words, in the example given, if the parttemperature does not exceed the desired value in the particular step inprogress, the logic checks whether the actual ambient temperatureexceeds with more than 20° F. the actual required part temperature. Ifthere is more than the permissible overshoot, the heating elements areshut off or decreased in output.

If on the other hand, no impermissible overshoot exists, the computerchecks whether the logic circuit regarding maximum allowable partdifferential (temperature lag logic) is disabled or not. Thecircumstances under which the temperature lag logic is disabled aredescribed below. If the temperature lag logic is not disabled, then themaximum temperature differential (Δ T_(p)) between any two points of thecomposite specimen 20 is compared to the maximum allowable differentialin the given temperature range. In the specific example given, if thecuring process is in its second step, and the average surface or parttemperature (T_(p)) is 210° F., the computer 56 checks whether Δ T_(p)exceeds 10° F. If Δ T_(p) does not exceed the maximum permissible value,then the heating is continued or turned on, as applicable. If, on theother hand, Δ T_(p) exceeds the maximum permissible value, then theheating is turned off or decreased.

As was noted above, the temperature lag logic may be disabled undercertain circumstances. This occurs, when the temperature lag logic andthe comparison of the actual part temperature to desired parttemperature result in conflicting commands. For example, comparison ofactual part temperature with the stored desired value of the parttemperature may require continued heating. At the same time, however,the temperature lag logic circuit may require that the heating bediscontinued. In such an event, the computer 56 is programmed todisregard the temperature lag logic circuit command and continueheating, albeit only at the minimum permissible rate. In the specificexample shown in Table I, this rate is 1° F./minute.

Still referring to FIG. 5, operation of the pressure control logic isexplained. The pressure control logic operates independently of thetemperature control logic, and determines whether the autoclave 34should be pressurized or kept at atmospheric pressure. Therefore, thepressure control logic in effect, operates the several vents and valveswhich introduce pressurized nitrogen or air into the autoclave 34, orvent the pressurized gas from the same. In this regard, it is noted thatthe pressure in the autoclave 34 may exceed a desired pressure not onlybecause of over pressurization, but also because the pressure of the gasin the autoclave 34 increases as the autoclave 34 is heated.

In accordance with one aspect of the present invention, the actualpressure or ambient pressure (P) inside of the autoclave 34 iscontinuously measured and compared to a predetermined desired pressureprofile which is stored in the computer's 56 memory. Furthermore, thevalves and vents are actuated to maintain the actual pressure in theautoclave 34 in close conformity with the predetermined profile. Thepredetermined pressure profile, however, may be a relatively simpleprofile. In the specific example given here, the pressure profilerequires either atmospheric or a fixed reference pressure, specifically85 PSI, in the autoclave 34. Thus, in the specific example given, thecomputer 56 is programmed to maintain atmospheric pressure in theautoclave 34 until 20 minutes have passed in step 3 of the curingprocess. At that point, 85 PSI should be applied and maintained until200° F. is reached in step 6, at which time the pressure should bevented. It is readily apparent from the foregoing description, thataccurate maintenance of even such a relatively simple pressure profileis not possible without continuous monitoring of the actual pressure inthe autoclave 34 and continuous use of the obtained data to governoperation of the pressurization and venting valves.

Still having specific reference to FIG. 5, a first pressure controllogic, having the continuous input of the pressure sensors 52 is shown.The pressure control logic first checks if the input of the pressuresensors 52 is manually overridden, i.e. disabled. If the pressuredcontrol logic is not manually disabled, the actual pressure (P) iscompared to a reference pressure (P₁). If P is higher than the referencepressure P₁, the venting mechanism is actuated. If P is lower than thereference pressure (P₁), the logic gives appropriate control commands tobring the pressure of the autoclave up to the reference pressure (P₁).In order to determine whether the pressurization should occur via air ornitrogen, the logic also compares the actual ambient pressure (P) toanother programmed reference pressure P_(o). If P is smaller than P_(o),the autoclave is pressurized with nitrogen gas, otherwise with air.

A second aspect of the pressure control logic is concerned with thedetermination of the reference pressure value P₁. In the hereindescribed specific embodiment, the pressure control logic first checkswhether the particular step in the curing process calls for venting ofthe autoclave. If the step calls for venting, the reference pressure P₁is considered to be zero (0), i.e. atmospheric pressure. If theparticular step of the curing process in progress does not call forventing, the pressure control logic checks whether the momentarilyexisting reference pressure P₁ is in fact the one called for by the stepin progress. If this is the case, the instantaneous value of P₁ ismaintained.

In the next step, however, the logic may depend on a real time input,viscosity input, or both, to determine the proper reference value (P₁)of the pressure. In accordance with one embodiment of the presentinvention, the logic may simply depend on the programmedpressure-versus-time profile to determine the correct referencepressure. For example, in the preferred embodiment specificallydescribed here, the logic, with the assistance of a real time clock 60,may establish that 20 minutes have elapsed in step 3 of the process, andtherefore P₁ should be adjusted to 85 PSI.

In another embodiment of the present invention, the logic evaluates theinput of the ultrasonic wave attenuation sensors 22, computes thecorresponding viscosity, and adjusts the pressure in accordance with aprogrammed pressure-versus-viscosity profile. In this regard, it isnoted that the viscosity of the resin substrate 20 is continuouslymonitored, and preferably permanently recorded during the entire curingprocess.

In still another embodiment of the present invention, the pressurecontrol logic, will adjust P₁ to a predetermined value, such as 85 PSI,provided the measured viscosity reaches a predetermined reference value.

In yet another preferred embodiment of the present invention, such asthe one specifically disclosed herein, the pressure control logic willincrement the reference pressure (P₁) to a predetermined value as soonas the predetermined reference viscosity is reached. If, however, thepredetermined reference viscosity is not reached prior to expiration ofa programmed time period, the programmed pressure-versus-time profileoverrides the programmed pressure-versus-viscosity profile, and thereference pressure is adjusted regardless of the actual viscosity datainput.

It is believed that a person having average skill in the art of computerprogramming is able to prepare an operational process control program onthe basis of the above description. Nevertheless, for the sake of fulland complete disclosure, a copy of the process control program of thehereinabove given specific example, is submitted simultaneously with thepresent application for patent. The simultaneously submitted program ishereby expressly incorporated by reference.

In addition to the hitherto enumerated components, the system 32 of thepresent invention includes an error check logic, and correspondingcircuitry 62. The error check logic and circuitry 62 checks, in effect,whether the several elements of the system 32 are functioning properly.For example, the error check logic and circuitry 62 checks ifpressurization and heating indeed occur when the appropriate commandsignals are given.

The several advantages of the above-described system of the presentinvention have already been described or alluded to in the foregoing. Insummary, the present system permits precise in-process control of thefinal curing step of composite materials based upon in-processmeasurements of vital physical parameters of the process and uponinstantaneous feedback to control said parameters. A process engineermay first empirically determine the optimal parameters of the curingprocess, and with the assistance of the present system 32 substantiallyobserve such parameters every time the curing process is performed.Several modifications of the present invention may become readilyapparent to those skilled in the art in light of the above disclosure.Therefore, the scope of the present invention should be determinedsolely from the following claims.

What is claimed is:
 1. A process for determining the dynamic viscosityof a stationary specimen of a polymeric resin which is subjected to atime-varying temperature, the process comprising the step of determiningthe amplitude attenuation of an ultrasonic wave in the polymeric resin,to obtain a value which has a linear relationship to the logarithm ofthe dynamic viscosity.
 2. The process of claim 1 wherein the step ofdetermining includes a step of passing an ultrasonic wave of a knownamplitude through the resin, and a step of sensing the amplitude of theultrasonic wave after it has travelled through the resin.
 3. The processof claim 2 wherein the frequency of the ultrasonic wave is in the0.1-2.5 MHz range.
 4. The process of claim 3 wherein the amplitudesensed is a root mean square amplitude of the ultrasonic wave.
 5. Theprocess of claim 2 wherein the polymeric resin is a binding resin of afiber-reinforced composite.
 6. A process for monitoring the viscosity ofa resin substrate of a fiber-reinforced composite material undergoing afinal curing step during which the composite material is subjected to atime-varying temperature, the process comprising the stepsof:irradiating the composite material with an ultrasonic wave, at leastthe root mean square amplitude of which is known; and sensing at leastthe root mean square amplitude of the ultrasonic wave after it hastravelled through at least a portion of the composite material, andthereby obtaining a value which has a linear relationship to thelogarithm of the instantaneous viscosity of the resin.
 7. The process ofclaim 6 wherein the ultrasonic wave is in the 0.1-2.5 MHz frequencyrange.
 8. The process of claim 6 wherein the process includes theadditional steps of positioning an ultrasonic wave source on a surfaceof the composite material and positioning an ultrasonic wave amplitudesensor on a surface of the composite material at a location remote fromthe location of said source.